1. Field of the Invention
The present invention relates to a communication network, a path setting method, and a recording medium having a path setting program recorded thereon, and more particularly to a communication network, a path setting method, and a recording medium having a path setting program recorded thereon in a mesh type communication network.
2. Description of the Related Art
Methods for forming a mesh type network in a public communication network includes a methods using a cross connecting unit by Synchronous Optical Network (SONET) or Synchronous Digital Hierarchy (SDH) technique, or a method using a cross connecting unit by Asynchronous Transfer Mode (ATM) technique, and various failure recovery schemes are proposed in each method. A failure recovery scheme proposed in T. Wu, “Fiber Network Service Survivability,” Artech House, 1992, Chapter 5 can be classified into a centralized control scheme and a decentralized control scheme. The centralized control scheme is a scheme for controlling almost all processing steps concerning failure recovery in a network or a subnetwork in one centralized control unit, and alarms indicating failure detection are once centralized in the centralized control unit, and then the centralized control unit determines a proceeding step to be next performed based on the alarms, and instructs several related nodes to perform the proceeding step to detour communication traffic from a link or a node where failure occurs. The decentralized control scheme is a scheme in which nodes constituting a network perform failure recovery processing in an autonomously decentralized manner.
The failure recovery scheme can be also classified into a preplanned scheme and a dynamic scheme. In the preplanned scheme, a route of an alternate path is previously calculated with respect to a primary path, and occurrence of failure immediately causes switch to the alternate path. In the dynamic scheme, a route of an alternate path is calculated after detection of failure, and finding the route causes switch to the alternate path. The preplanned scheme is classified into a 1+1 and 1:1 scheme in which one alternate path is prepared with respect to one primary path, 1:n scheme in which n (an integer not less than 2) primary paths share one alternate path source, and m:n scheme in which n primary paths share m (an integer not less than 2) alternate path sources. Sharing the alternate path source achieves an advantage of increased use efficiency of the source. However, a conflict may occur in such a manner that the plurality of primary paths try to take one alternate path source when multiple failures occur, so that caution must be taken in determining which primary paths share the alternate path source.
Attention has been recently given to technique of applying an improved protocol that is developed for an Internet Protocol (IP) network to an optical network to achieve high speed provisioning and high speed failure recovery of an optical path in the optical network. For example, in Multi-Protocol Label Switching (MPLS) working group of Internet Engineering Task Force (IETF), standardization of control technique of such an optical network is implemented. In an internet draft: draft-many-ip-optical framework-01.txt submitted to the IETF, a concept of Shared Risk Link Group (SRLG) is introduced for facilitating route calculation of an alternate path in failure recovery. The SRLG is a group consisting of a plurality of links sharing the same physical source. Sharing the same physical source means that all the links belonging to the SRLG are affected when failure occurs in the shared physical source. For example, a plurality of optical fibers in the same pipe are simultaneously affected by one failure of cutting of the pipe. In a wavelength division multiplexed optical network, cutting of one optical fiber affects a plurality of wavelengths in the optical fiber. The SRLG is identified by SRLG ID and used in the route calculation of the alternate path. For example, in page 24 of draft-many-ip-optical framework-01.txt, it is described that a primary path and an alternate path should be adapted not to pass links belonging to the same SRLG in 1+1 failure recovery. In page 26 of draft-many-ip-optical framework-01.txt, it is described that alternate paths corresponding to a plurality of primary paths should be able to share one link simply when the plurality of primary paths do not pass links belonging to the same SRLG.
In this application, a group of sources sharing a risk such as SRLG is referred to as a risk sharing resource group. Setting paths in consideration of the risk sharing resource group can prevent a plurality of paths from being simultaneously affected by one failure to make failure recovery impossible.
Examples of path setting methods of this kind are described in National Publication of International Patent Application No. 11-508421, Japanese Patent Laid-Open No. 9-224026 and Japanese Patent No. 2770749.
To calculate the primary path or alternate path in consideration of the risk sharing resource group, it is necessary to know to which risk sharing resource group the source such as the link or node in the network belongs. Specifically, in the decentralized control scheme, all nodes must perform route calculation to thereby respectively have risk sharing resource group information of the entire network. With increasing size of the network, the amount of information becomes enormous to require that each node has a large amount of memory source.
When a link of alternate paths corresponding to a plurality of primary paths are to be shared, in order to determine whether a link used in calculation of an alternate path corresponding to a certain primary path can be shared with alternate paths corresponding to other primary paths, it is also necessary to know which link other all primary paths and alternate paths corresponding thereto pass. Path information is often updated in accordance with set or release of the path, so that there is a problem that when all nodes have the pass information of the entire network in the decentralized control scheme, traffic for transmission of the pass information between the nodes significantly increases. This problem also becomes more noticeable with increasing size of the network.
One of means for solving the problems is, as described in page 26 of draft-many-ip-optical framework-01.txt, to prepare a route server having all necessary information such as topology information of a network, information on a risk sharing resource group, or path information, and to perform route calculation of the primary path and alternate path by the centralized control scheme. However, the centralized control scheme has the following problems: 1) when the size of the network is large, load of route calculation centralized in a route server becomes too large; 2) when failure occurs in the route server, route calculation can be no longer performed to cause lower failure resistance than the decentralized control scheme.
Specifically, in the conventional techniques, there are two path setting methods:
(A) a method for imparting all information to the nodes; and
(B) a method for imparting all information to the centralized control unit. However, (A) has a problem that traffic for synchronizing the information between the nodes becomes enormous, and (B) has a problem that load of calculation is centralized in the centralized control unit.